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  • 1.
    Alves, Luis
    et al.
    Univ Coimbra, Coimbra, Portugal.
    Medronho, Bruno
    Univ Algarve, Faro, Portugal.
    Filipe, Alexandra
    Univ Algarve, Faro, Portugal.
    Antunes, Filipe E.
    Univ Coimbra, Coimbra, Portugal.
    Lindman, Björn
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering. Univ Coimbra, Coimbra, Portugal; Lund Univ, Lund.
    Topgaard, Daniel
    Lund Univ, Lund.
    Davidovich, Irina
    Technion Israel Inst Technol, Haifa, Israel.
    Talmon, Yeshayahu
    Technion Israel Inst Technol, Haifa, Israel.
    New Insights on the Role of Urea on the Dissolution and Thermally-Induced Gelation of Cellulose in Aqueous Alkali2018In: GELS, ISSN 2310-2861, Vol. 4, no 4, article id 87Article in journal (Refereed)
    Abstract [en]

    The gelation of cellulose in alkali solutions is quite relevant, but still a poorly understood process. Moreover, the role of certain additives, such as urea, is not consensual among the community. Therefore, in this work, an unusual set of characterization methods for cellulose solutions, such as cryo-transmission electronic microscopy (cryo-TEM), polarization transfer solid-state nuclear magnetic resonance (PTssNMR) and diffusion wave spectroscopy (DWS) were employed to study the role of urea on the dissolution and gelation processes of cellulose in aqueous alkali. Cryo-TEM reveals that the addition of urea generally reduces the presence of undissolved cellulose fibrils in solution. These results are consistent with PTssNMR data, which show the reduction and in some cases the absence of crystalline portions of cellulose in solution, suggesting a pronounced positive effect of the urea on the dissolution efficiency of cellulose. Both conventional mechanical macrorheology and microrheology (DWS) indicate a significant delay of gelation induced by urea, being absent until ca. 60 degrees C for a system containing 5wt % cellulose, while a system without urea gels at a lower temperature. For higher cellulose concentrations, the samples containing urea form gels even at room temperature. It is argued that since urea facilitates cellulose dissolution, the high entanglement of the cellulose chains in solution (above the critical concentration, C*) results in a strong three-dimensional network.

  • 2.
    Lindman, Björn
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering. Lund Univ, Ctr Chem & Chem Engn, Dept Chem, Div Phys Chem, SE-22100 Lund, Sweden.
    Medronho, Bruno
    Univ Algarve, Fac Sci & Technol, P-8005139 Faro, Portugal..
    The Subtleties of Dissolution and Regeneration of Cellulose: Breaking and Making Hydrogen Bonds2015In: BioResources, ISSN 1930-2126, E-ISSN 1930-2126, Vol. 10, no 3, p. 3811-3814Article in journal (Other academic)
    Abstract [en]

    Cellulose dissolution and regeneration are old topics that have recently gained renewed attention. This is reflected in both applications - earlier and novel - and in scientific controversies. There is a current discussion in the literature on the balance between hydrogen bonding and hydrophobic interactions in controlling the solution behavior of cellulose. Some of the key ideas are recalled.

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